INSTRUCTION DESCRIPTIONS
A - 72 INSTRUCTION SET DETAILS MOTOROLA
Operation: Assembler Syntax:
S2 – S1(parallel move) CMP S1, S2 (parallel move)
Description: Subtract the source one operand, S1, from the source two accumulator,
S2, and update the condition code register. The result of the subtraction operation is not
stored.
Note: This instruction subtracts 56-bit operands. When a word is specified as S1, it is
sign extended and zero filled to form a valid 56-bit operand. For the carry to be set cor-
rectly as a result of the subtraction, S2 must be properly sign extended. S2 can be
improperly sign extended by writing A1 or B1 explicitly prior to executing the compare so
that A2 or B2, respectively, may not represent the correct sign extension. This note par-
ticularly applies to the case where it is extended to compare 24-bit operands such as X0
with A1.
Example: :
CMP Y0,B X0,X:(R6)+N6 Y1,Y:(R0)– ;comp. Y0 and B, save X0, Y1
:
Explanation of Example: Prior to execution, the 56-bit B accumulator contains the
value $00:000020:000000 and the 24-bit Y0 register contains the value $000024. The
execution of the CMP Y0,B instruction automatically appends the 24-bit value in the Y0
register with 24 LS zeros, sign extends the resulting 48-bit long word to 56 bits, subtracts
the result from the 56-bit B accumulator and updates the condition code register.
CMP Compare CMPBefore Execution After Execution
B B
$00:000020:000000
Y0 Y0
$000024 $000024
$00:000020:000000
SR SR
$0300 $0319